1. Field of the Invention
The invention relates to an information recording/reproducing device for recording and reproducing information in a recording medium by using a conductive probe.
2. Description of the Related Art
Recently, small-sized portable appliances are spreading widely around the world, and as high-speed information transmission networking has notably advanced, the demand is rapidly mounting for nonvolatile memories of small size and large capacity. Among them, the NAND-type flash memory and a small-sized hard disk drive (HDD) have rapidly advanced in recording density, and are forming a large market.
Against this background, new memories intended to substantially surpass the limits of recording density are being developed, and results of research and development of recording and reproducing systems based on new theories have been recently reported inside and outside Japan.
One of the most noteworthy recording and reproducing systems is the resistive RAM (ReRAM) making use of resistance changes due to application of current pulses to a recording layer (see, for example, T. Gotoh, K. Sugawara and K. Tanaka, Jpn. J. Appl. Phys., 43, 6B, 2004, L818, and A. Sawa, T. Fuji, M. Kawasaki and Y. Tokura, Appl. Phys. Lett., 85, 18, 4073, 2004).
A recording layer is composed of, for example, a two-element metal oxide such as TiO2, CuxO, and Fe2O3, and a composite oxide such as Pr(1-x)CaxMnO3, SrTiO3 doped with Cr, and SrZrO3 doped with Cr. A composition of forming a recording layer from a metal chalcogenide compound such as Cu2S is called a programmable metallization cell (PMC).
In the ReRAM, by applying a voltage to the recording layer, a current passage of low resistance and substantially having a metallic electrical conductivity is formed in the recording layer. As a result, the resistance of the recording layer drops. If such current passage is not formed, on the other hand, the recording layer is an insulator or a semiconductor, and its resistance is high.
The difference in such resistances, that is, the on/off ratio, reaches as high as 103 or more.
Therefore, when a reading current is applied to the recording layer, a voltage difference according to the resistance difference of the recording layer is generated, and by detecting this voltage difference, reproduction at high sensitivity may be realized.
In this recording and reproducing system, by reducing the size of the recording layer, the current necessary for recording is decreased, which contributes to advancement of density.
There are other examples of memories suited to recording and reproducing at high density, such as probe memories having a cantilever array formed by the technology of micro-electromechanical systems (MEMS) (see, for example, P. Vettiger, G. Cross, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz and G. K. Binning, IEEE Trans. Nanotechnology 1, 39 (2002), and P. Vettiger, T. Albrecht, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, D. Jubin, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz, D. Wiesman and G. K. Binning, P. Bachtold, G. Cherubini, C. Hagleitner, T. Loeliger, A. Pantazi, H. Pozidis and E. Eleftheriou, in Technical Digest, IEDMO3, pp. 763-766).
A major feature of the probe memory is that the recording density can be enhanced outstandingly because wiring is not needed in each recording area for recording bit data.
In the probe memory, the probe is set in contact with the recording layer composed of, for example, an organic polymer material, and information is thermally recorded topographically in part (bit) of the recording layer. When reproducing, the resistance change of the cantilever resistor occurring depending on presence or absence of bit recording is detected.
In the driving system of such probe memory, mainly, about 1,000 cantilevers (probes) are provided on one chip, and they are driven in parallel. The cantilevers on one chip can be formed simultaneously by the MEMS technology.
According to a demonstration by a single probe, the recording density of the probe memory has been already proved to be as high as 1.14 Tb/in2, far higher than the recording density of a hard disk drive (HDD).
The probe memory is expected to be used as a mobile storage in the future, but assuming application in a memory card, a great demerit is that the transfer speed is less than 1/10 as compared with the existing HDD. By thermal recording, another problem is the possibility of increase of power consumption along with enhancement of density.
Accordingly, instead of thermal topographic recording in polymer, the ReRAM recording and reproducing system is expected to be capable of recording and reproducing at high speed and at lower power consumption, and when it is applied to the information recording/reproducing device such as HDD, an ideal memory characterized by high speed, low power consumption, and high density may be realized.
In this case, however, a probe position control system of high precision must be established.